The invention is directed to a bath for the electrolytic (galvanic) deposition of gold-indium alloy coatings at a pH of less than 3 consisting of (or consisting essentially of) 1 to 20 grams/l of gold in the form of alkali and/or ammonium tetracyanoaurate (III), 0.5 to 50 g/l of indium in the form of a water soluble indium salt, an acid and a buffer or conducting salt.
The co-deposition of indium from electrolytic gold electrolytes leads to light yellow gold coatings which above all are used in the decorative industry in gold plating watch cases, arm bands, eyeglass frames, or jewelry. In addition to good general corrosion resistance in comparison to other gold alloy coatings, the coatings have an especially good resistance to the gradual corrosion by silver sulfide. The gold-indium alloy coatings which deposit from weakly acid baths at pH 3.5-5 (e.g. German Pat. No. 1111897) are characterised by great brittleness and are inclined to the formation of fissures through which the resistance to corrosion is greatly impaired. Therefore indium is deposited together with other metals such as nickel or cobalt which impair the resistance to gradual corrosion by silver sulfide. The indium content in the coating is only about 1%.
According to German Pat. No. 3,012,999 (and related Zilske U.S. Pat. No. 4,391,679, the entire disclosure of which is hereby incorporated by reference and relied upon) the co-deposition of indium from a strongly acid gold electrolyte based on potassium tetracyanoaurate (III) is possible. Under the stated conditions ductile non-glossy coatings are obtained, but no glossy coatings are obtained with indium contents of 2-3%. By the addition of a nickel or cobalt salt to the electrolyte it is true that a glossy coating deposits, which, however, has a relatively strong gray appearance. A light yellow color tone is not produced. Besides in this case the resistance to gradual corrosion is also reduced.
These baths contain 1 to 20 g/l of gold in the form of tetracyanoaurate (III), a water soluble alloying metal salt, an acid and a complex former at a pH between 0.4 and 2.5.